The present invention relates to a photocurable and thermosetting resist ink composition, which is developable with a diluted, alkaline solution, to a cured resist film formed on the printed circuit board by using the composition, to a printed circuit board having thereon a solder resist produced by using the composition, and also to a process for producing the printed circuit board.
Recently, for the method of forming resist patterns for various printed circuit boards for consumer use and industrial use, a dry film or liquid developable resist ink having a high resolution and dimensional accuracy has come to be employed essentially, in place of screen printing systems, in order to meet the recent demand for printed wiring boards with high wiring densities.
Regarding this situation, disclosed in Japanese Patent Publication Nos. 56-40329/1981 and 57-45785/1982 is a resist ink composition consisting essentially of a reaction product to be obtained by reacting an epoxy resin with an unsaturated monocarboxylic acid followed by adding thereto a polybasic acid anhydride. In U.S. Pat. No. 5,009,982, disclosed is a photosensitive resin composition for a resist ink which is developable with a diluted alkali and which comprises a resin to be obtained by adding an acid anhydride to a novolak epoxy acrylate (this resin is soluble in a diluted, alkaline aqueous solution), a photopolymerization initiator, a diluent, and a thermosetting component comprising an epoxy compound. The composition comprising such an adduct composed of a novolak epoxy acrylate and a polybasic acid anhydride finally gives a relatively good cured resist film but is defective in that the epoxy acrylate-acid anhydride adduct does not always have sufficient photocurability at low exposure since the molecular weight of the raw material novolak epoxy resin available for industrial use is limited rather low.
In order to overcome this drawback, for example, considered may be the use of a carboxyl-containing polymer of ethylenic unsaturated monomers, such as (meth)acrylic or styrenic monomers, having a molecular weight of approximately from 10,000 to 200,000, as the polymer binder, while the photocurability of the composition is derived only from the diluent such as acrylic monomers. In such a resist ink composition, however, the polymer binder itself does not exhibit photocurability. Therefore, the composition still does not have sharp photocurability and often involves problems in that it is easily swollen or peeled during development with a diluted alkali.
When the resist ink composition is applied on a substrate with a spray coater, its viscosity must be lowered so as to make the coating operation easy. For this, in general, a large amount of a diluent is added to the composition thereby lowering its viscosity. If so, however, the components in the composition which are hardly soluble in the diluent added, such as pigments, epoxy resins, etc. often precipitate and separate out, and therefore the dispersion stability of the composition must be improved.